The Department of Defense (DoD,[3]USDOD, or DOD) is an executive branch department of the federal government of the United States charged with coordinating and supervising all agencies and functions of the government concerned directly with national security and the United States Armed Forces. The department is the largest employer in the world,[4] with nearly 1.3 million active duty servicemen and women[a] as of 2016.[5] Adding to its employees are over 826,000 National Guardsmen and Reservists from the four services,[b] and over 742,000 civilians bringing the total to over 2.8 million employees.[1] Headquartered at the Pentagon in Arlington, Virginia, just outside Washington, D.C., the DoD's stated mission is to provide "the military forces needed to deter war and ensure our nation's security".[6][7]

The history of the defense of the United States started with the Continental Congress in 1775. The creation of the United States Army was enacted on 14 June 1775.[8] This coincides with the American holiday Flag Day. The Second Continental Congress would charter the United States Navy, on 13 October 1775,[9] and create the United States Marine Corps on 10 November 1775.

The Preamble of the United States Constitution gave the authority to the federal government to defend its citizens:

We the People of the United States, in Order to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defence, promote the general Welfare, and secure the Blessings of Liberty to ourselves and our Posterity, do ordain and establish this Constitution for the United States of America.

— Constitution of the United States

Upon the seating of the first Congress on 4 March 1789, legislation to create a military defense force stagnated as they focused on other concerns relevant to setting up the new government. President George Washington went to Congress to remind them of their duty to establish a military twice during this time. Finally, on the last day of the session, 29 September 1789, Congress created the War Department, historic forerunner of the Department of Defense.[10][11] The War Department handled naval affairs until Congress created the Navy Department in 1798. The secretaries of each of these departments reported directly to the president as cabinet-level advisors until 1949, when all military departments became subordinate to the Secretary of Defense.

President Harry Truman signs the National Security Act Amendment of 1949

After the end of World War II, President Harry Truman proposed creation of a unified department of national defense. In a special message to Congress on 19 December 1945, the President cited both wasteful military spending and inter-departmental conflicts. Deliberations in Congress went on for months focusing heavily on the role of the military in society and the threat of granting too much military power to the executive.[12]

Under the Department of Defense Reorganization Act of 1958 (Pub.L. 85–599), channels of authority within the department were streamlined, while still maintaining the ordinary authority of the Military Departments to organize, train and equip their associated forces. The Act clarified the overall decision-making authority of the Secretary of Defense with respect to these subordinate Military Departments and more clearly defined the operational chain of command over U.S. military forces (created by the military departments) as running from the president to the Secretary of Defense and then to the unified combatant commanders. Also provided in this legislation was a centralized research authority, the Advanced Research Projects Agency, eventually known as DARPA. The act was written and promoted by the Eisenhower administration, and was signed into law 6 August 1958.

The Secretary of Defense, appointed by the president with the advice and consent of the Senate, is by federal law (10 U.S.C.§ 113) the head of the Department of Defense, "the principal assistant to the President in all matters relating to Department of Defense", and has "authority, direction and control over the Department of Defense". Because the Constitution vests all military authority in Congress and the president, the statutory authority of the Secretary of Defense is derived from their constitutional authorities. Since it is impractical for either Congress or the president to participate in every piece of Department of Defense affairs, the Secretary of Defense, and the secretary's subordinate officials generally, exercise military authority.

Department of Defense Directive 5100.01 describes the organizational relationships within the department, and is the foundational issuance for delineating the major functions of the department. The latest version, signed by former Secretary of Defense Robert Gates in December 2010, is the first major re-write since 1987.[17][18]

The Office of the Secretary of Defense (OSD) is the secretary and deputy secretary's (mainly) civilian staff.

OSD is the principal staff element of the Secretary of Defense in the exercise of policy development, planning, resource management, fiscal and program evaluation and oversight, and interface and exchange with other U.S. Government departments and agencies, foreign governments, and international organizations, through formal and informal processes. OSD also performs oversight and management of the Defense Agencies and Department of Defense Field Activities.

Following the Goldwater-Nichols Act in 1986 the Joint Chiefs of Staff do not have operational command authority, neither individually nor collectively, as the chain of command goes from the president to the Secretary of Defense, and from the Secretary of Defense to the commanders of the Combatant Commands.[24] Goldwater-Nichols also created the office of vice-chairman, and the chairman is now designated as the principal military adviser to the Secretary of Defense, the Homeland Security Council, the National Security Council and to the president.[25]

The secretaries of the Military Departments and the Service Chiefs do not possess operational command authority over U.S. troops (this power was stripped from them in the Defense Reorganization Act of 1958), and instead the Military Departments are tasked solely with "the training, provision of equipment, and administration of troops."[29]

A Unified Combatant Command is a military command composed of personnel and equipment from at least two Military Departments, which has a broad and continuing mission.[30][31]

The Military Departments are responsible for equipping and training the troops to fight, while the Unified Combatant Commands are responsible for actual operational command of military forces.[31] Almost all operational U.S. forces are under the authority of a Unified Command.[29] The Unified Commands are governed by a Unified Command Plan, a frequently updated document (produced by the Department of Defense) which lays out the Command's mission, geographical/functional responsibilities, and force structure.[31]

Department of Defense spending in 2010 was 4.8% of GDP and accounted for about 45% of budgeted global military spending – more than the next 17 largest militaries combined.[33][34]

The Department of Defense accounts for the majority of federal discretionary spending. In FY 2010 the Department of Defense budgeted spending accounted for 21% of the U.S. Federal Budget, and 53% of federal discretionary spending, which represents funds not accounted for by pre-existing obligations.[35] However, this does not include many military-related items that are outside the Defense Department budget, such as nuclear weapons research, maintenance, cleanup, and production, which is in the Department of Energy budget, Veterans Affairs, the Treasury Department's payments in pensions to military retirees and widows and their families, interest on debt incurred in past wars, or State Department financing of foreign arms sales and militarily-related development assistance. Neither does it include defense spending that is not military in nature, such as the Department of Homeland Security, counter-terrorism spending by the FBI, and intelligence-gathering spending by the NSA.

Chart showing growth in U.S. Department of Defense spending (2000–2011)

In the 2010 United States federal budget, the Department of Defense was allocated a base budget of $533.7 billion, with a further $75.5 billion adjustment in respect of 2009, and $130 billion for overseas contingencies.[36] The subsequent 2010 Department of Defense Financial Report shows the total budgetary resources for fiscal year 2010 were $1.2 trillion.[37] Of these resources, $1.1 trillion were obligated and $994 billion were disbursed, with the remaining resources relating to multi-year modernization projects requiring additional time to procure.[37] After over a decade of non-compliance, Congress has established a deadline of Fiscal year 2017 for the Department of Defense to achieve audit readiness.[38]

In 2015, a Pentagon consulting firm performed an audit on the Department of Defense's budget. It found that there was $125 billion in wasteful spending that could be saved over the next five years without layoffs or reduction in military personnel. In 2016, The Washington Post uncovered that rather than taking the advice of the auditing firm, senior defense officials suppressed and hid the report from the public to avoid political scrutiny.[42]

The Department of Defense was the largest single consumer of energy in the United States in 2006.[44]

In FY 2006, the department used almost 30,000 gigawatt hours (GWH) of electricity, at a cost of almost $2.2 billion. The department's electricity use would supply enough electricity to power more than 2.6 million average American homes. In electricity consumption, if it were a country, the department would rank 58th in the world, using slightly less than Denmark and slightly more than Syria (CIA World Factbook, 2006).[45]

The Department of Defense was responsible for 93% of all US government fuel consumption in 2007 (Department of the Air Force: 52%; Department of the Navy: 33%; Department of the Army: 7%; other Department components: 1%).[45] The Department of Defense uses 4,600,000,000 US gallons (1.7×1010 L) of fuel annually, an average of 12,600,000 US gallons (48,000,000 L) of fuel per day. A large Army division may use about 6,000 US gallons (23,000 L) per day. According to the 2005 CIA World Factbook, if it were a country, the Department of Defense would rank 34th in the world in average daily oil use, coming in just behind Iraq and just ahead of Sweden.[46] The Air Force is the largest user of fuel energy in the federal government. The Air Force uses 10% of the nation's aviation fuel. (JP-8 accounts for nearly 90% of its fuels.) This fuel usage breaks down as such: 82% jet fuel, 16% facility management and 2% ground vehicle/equipment.[47]

In the latest Center for Effective Government analysis of 15 federal agencies which receive the most Freedom of Information Act (United States) (FOIA) requests, published in 2015 (using 2012 and 2013 data, the most recent years available), the DoD earned a D− by scoring 61 out of a possible 100 points, i.e. did not earn a satisfactory overall grade. While it had improved from a failing grade in 2013, it still had low scores in processing requests (55%) and their disclosure rules (42%).[48]

^Joe Carmel, ed. (n.d.) [Original Statute 1789]. "Statutes at Large, Session I, Charter XXV"(PDF). Legisworks. Retrieved 28 January 2018. An Act to recognize and adapt to the Constitution of the United States the establishment of the Troops raised under the Resolves of the United Stales in Congress assembled, and for other purposes therein mentioned.